Many attempts have been made to improve the pigment dispersibility in a resin such as in paints, electrophotographic developers, inks, etc. For instance, there has been proposed a simple treating method using a coupling agent such as silane or titanate in Japanese Patent Publication No. 32415/73, Japanese Patent Application (OPI) No. 7648/83 (the term "OPI" as used herein means a "published unexamined Japanese patent application), and Terada and Murashige et al (Journal of Colorant Association, Vol. 51, pages 465 to 472 (1978)). Further, there has been proposed (1) a method of grafting a polymer to the surface of carbon black, by Ohkita, et al. (Journal of Colorant Association, Vol. 41, p. 187 (1968)); and a method of coating the surface of inorganic particles (silica) with a polymer (polystyrene) by means of emulsion polymerization, by Furusawa et al. (Kobunshi Ronbunshu, Vol. 40, No. 10, pages 697 to 702 (1983)).
However, although the treatment with the coupling agent may be satisfactory for use in several fields, it cannot be generally applied to a wide range of applications. Further, the method proposed by Ohkita, et al. may contribute to the improvement of the dispersibility in the resin by the polymer grafting, but this involves a drawback in that a satisfactory result cannot be obtained in the case where pigment particles have to be coated completely with the polymer. As another drawback, there has been a problem upon applying the method to various pigments other than carbon black. That is, it is impossible to use the method for treating various kinds of pigments by the polymer of an identical composition (particularly in the color toner pigment). While on the other hand, although polymer-coated particles can be obtained by the method of Furusawa, et al., the method has the drawback that particles comprising only polymer, and not containing inorganic particles, may be also formed. Further, since the stability of the emulsion is not sufficient in this method, it has the drawback that coagulated lumps may be caused partially.